Conventional process assembly lines consist of a number of stationary idle and work stations, each of which perform various loading, joining or sealing operations simultaneously. These work stations are generally connected by a reciprocating, straight-through shuttle line. The shuttle receives all of the product, transfers them from one station to the next synchronously, and a single, pre-determined series of operations is performed on each product as it is shuttled through the line.
This type of assembly line concept is best suited for an undifferentiated product, and is not conducive to model or product mix or any type of variety in the product. An example of model or product mix may be as dramatic as truck bodies and car bodies in an automotive assembly line, or as subtle as a new (or prototype) model of the same style car. Similarly, it is contemplated that the system may be used to manufacture products as disparate as snow-blowers and tractors tractor the same line. Typical synchronous assembly lines require major and expensive rework to accommodate such model mix, if changeover is at all feasible.
Capital expenditures, operating costs, floor space, and product mix have made it increasingly necessary to provide flexible assembly systems. However, many of the sophisticated "factory of the future" concepts implemented to date require expensive maintenance, and a high level of technically qualified personnel to operate them. Such systems often defeat the economic and technological advantages they at first seem to provide. Robotic vehicles, A.G.V. (automatically guided vehicles), flex-systems, etc., although successful in some operations, have not always proved practical and or maintainable for many manufacturing processes. High degrees of maintenance, frequent operator intervention, high degree of training for operator and maintenance personnel, and total cost reduce the desirability of such systems. Moreover, there remains a proliferation of assembly plants that, because of age, floor space, working capital, and available labor force, cannot utilize these technologies.
Quality is usually a paramount consideration and has become a critical issue in many automotive companies. Therefore, many manufacturers will insist that each sub-assembly be processed in the same work station in order to minimize set-up and to guarantee repeatability. Previous systems containing palletized work stations, wherein each sub-assembly is built up on a different tooling pallet (carousels, turntables, A.G.V's and palletized conveyors) have not been accepted, even though they are flexible, since they do not assure such repeatability.
Production rates also play a role in determining the type of system to be employed. When a low volume model run is considered, many of the available processes are not economical. Further, these manufactures face the added economic problems of model mix and tool changeovers.